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Published by Communications and Public Affairs (519) 824-4120, Ext. 56982 or 53338

News Release

August 12, 2003

Voluntary vaccination policies would reduce immunization rates, study says

Voluntary vaccination policies for diseases such as smallpox could result in increased mortality rates, new research by a University of Guelph mathematics professor reveals.

In a study published this week in Proceedings of the National Academy of Sciences, Chris Bauch says that self-interest would prevail over group-interest with voluntary vaccination policies, making it difficult to maintain persistently high levels of immunization. For example, Bauch and researchers from McMaster University and the University of California at Berkeley showed that making smallpox vaccinations voluntary in the United States could result in a significant increase in expected mortality – an average of 22 per cent – after an outbreak.

“Self-interest appears to impede voluntary vaccination policies,” said Bauch, who joined the U of G faculty in July and conducted the research as a post-doctoral researcher at McMaster University. He chose to focus on the U.S. small pox vaccination policy “because it has reemerged recently as a public health issue. The threat of bioterrorism has fueled debate on the policy in the U.S., with some proposals calling for voluntary mass vaccination.” Currently, the U.S. policy calls for mandatory smallpox vaccination of its 500,000 military personnel and voluntary vaccination of up to 10 million "first responders,” mostly health care workers. Starting in 2004, voluntary vaccination will also be available to the general public.

“Because of the lack of information on smallpox transmission in contemporary populations, mathematical modeling has an especially important role to play in policy development,” Bauch said. To compare the U.S. population's optimal vaccination level with the level that might actually be achieved under a voluntary program, he and his colleagues performed a first-ever mathematical analysis of the U.S. smallpox vaccination policy using game theory and epidemic modeling. Game theory is a mathematical analysis of how individuals make decisions in groups when the impact of their decision depends on the decisions reached by others in the group.

“If individuals decide whether to vaccinate according to self-interest, the level of immunity achieved may differ from that which is best for the population as a whole,” Bauch said. “This potential dilemma motivated our investigation and our game-theoretical model formalized this conflict between self-interest and group-interest.”

Bauch and the researchers analyzed a large number of plausible scenarios for smallpox outbreaks in the United States. For most of these scenarios, the findings were the same: making smallpox vaccinations strictly voluntary increased expected mortality rates. “All else being equal, self-interested decision-making can cause vaccine coverage levels to fall short,” he said.

The game theoretical approach is applicable to vaccination policies for other diseases as well, Bauch said. He hopes to build more realism and predictive power into the model for future research. “This analysis highlights the potential value of game theory in the evaluation and development of public health policy.”

The Proceedings of the National Academy of Sciences is one of the world's most cited multi-disciplinary scientific serials. Established in 1914, it publishes cutting-edge research reports, commentaries, reviews, perspectives and colloquium papers in the biological, physical, and social sciences.

Prof. Chris Bauch
Department of Mathematics and Statistics
(519) 824-4120, Ext. 53079

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